1. Field of the Invention
The present invention generally relates to a dual gate oxide high-voltage semiconductor (SOI) device. More particularly, the present invention relates to a high-voltage semiconductor device, such as a lateral MOSFET or a diode, having two gate oxides for optimizing breakdown voltage and specific-on-resistance.
2. Background Art
In fabricating high-voltage power devices, tradeoffs and compromises must often be made in areas such as breakdown voltage, specific-on resistance, size, conduction losses, manufacturing simplicity, and reliability. Frequently improvement of one characteristic such as breakdown voltage can lead to degradation in another such as specific-on-resistance. For example, to reduce the specific-on-resistance of a device, silicon doping is often increased. However, an increase in doping causes an increase in a magnitude of the electric field, which degrades the breakdown voltage of the device. Thus, the specific-on-resistance and breakdown voltage can be competing concerns.
Heretofore, several attempts have been made to improve the basic SOI structure. U.S. Pat. Nos. 5,246,870 and 5,300,448, both commonly assigned with the present application and herein incorporated by reference, attempt to improve breakdown voltage by providing a linear doping profile in the drift region. Specifically, these SOI devices include a drift region positioned between a body region and a drain region. The drift region is provided with various features such as a thinned portion and a linear lateral doping intensity profile in an attempt to provide increased breakdown voltage. However, to maintain high breakdown voltage, the total amount of conduction charge near the source side of the drift region must be kept very small. This often leads to bottlenecking for current flow, and preventing optimum reduction in conduction losses.
Another improvement over the basic SOI structures is shown in U.S. Pat. Nos. 5,969,387 and 6,221,737, both commonly assigned with the present application and herein incorporated by reference. These references disclose a SOI device (and method for forming the same) having a graded top oxide and drift region in an attempt to yield a better tradeoff between breakdown voltage and saturation current. However, the formation of the graded top oxide and drift region of these references relies upon a two-dimensional oxidation process in which an oxidation mask is patterned with a series of sequential openings.
In view of the foregoing, a need exists for a high-voltage SOI device in which both breakdown voltage and specific on-resistance can be optimized. A further need exists for a high-voltage SOI device having a dual gate oxide so that doping in the silicon can be increased without increasing the magnitude of the vertical electric field.